Nick,
Two different devices. Two different types of use. The WAV and DOS are their own independent AquaBus devices with their own independent power sources. Both can be set to have a "fallback" state. That is, the mode they run in when they lose communication from the base unit. Now, with the WAV it is easy, you may just want them to go to 50% power on loss of communication. For the DOS, since it might be putting stuff into the tank - and maybe based on other feedback (like pH or a switch), you might set that one to fallback OFF.
Definitely when you have a lot of your tank controlled by an Apex it is a good idea to have a back up Apex "brain". If you do have a backup, it will take a call to support for you to make the switchover to keep all your programming. Since it is not financially feasible for everyone to have a spare, I often recommend the local club have one or two on hand just for this purpose - so the person can have their tank running fine until they get their Apex replaced or back from the factory.
This is a good question, and I don't mind answering it here, but I would encourage you go to the
http://forum.neptunesystems.com for more input and advice from others on this matter as well.